Package Signature Checking During Installation

Summary

One long-standing problem in Fedora is that we don't check package signatures during installation. This has been a persistent issue since the very beginning of Fedora (and even in Red Hat Linux before it.) The reason for this has always been that there's no way to form any root of trust for the signatures in the repositories, and thus no reason they wouldn't have been modified along with whatever package would need to be re-signed after tampering.

Following the implementation of Features/SecureBoot, we can extend the Secure Boot keys as a root of trust provided by the hardware against which we can verify a signature on our key files, thus guaranteeing that they're from the same source as the boot media.